The present invention relates to rotary electric motors, more particularly to motors having a plurality of permanent magnet rotor elements and magnetically isolated stator elements with a plurality of air gaps between the stator and rotor elements.
The progressive improvement of electronic systems, such as microcontroller and microprocessor based applications for the control of motors, as well as the availability of improved portable power sources, has made the development of more efficient electric motor drives a compelling challenge. Electronically controlled pulsed energization of windings of motors offers the prospect of more flexible management of motor characteristics. By control of pulse width, duty cycle, and switched application of a battery source to appropriate stator windings, functional versatility that is virtually indistinguishable from alternating current synchronous motor operation can be achieved. The use of permanent magnets in conjunction with such windings is advantageous in limiting current consumption.
The above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, identifies and addresses the need for an improved motor amenable to simplified manufacture and capable of efficient and flexible operating characteristics. It is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. The copending related U.S. applications incorporate electromagnet poles as isolated magnetically permeable structures configured in an annular ring, relatively thin in the radial direction, to provide advantageous effects. With this arrangement, flux can be concentrated, with virtually no loss or deleterious transformer interference effects in the electromagnet cores, as compared with prior art embodiments. While improvements in torque characteristics and efficiency are attainable with the structure of the identified copending application, further improvements remain desirable.
The Maslov et al. applications recognize that isolation of the electromagnet groups permits individual concentration of flux in the magnetic cores of the groups, with low flux loss and no deleterious transformer interference effects with other electromagnet members. Operational advantages can be gained by configuring a single pole pair as an isolated electromagnet group. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched.
Copending related U.S. patent application of Maslov et al., Ser. No. 09/966,101 describes benefits to be gained from utilization of three dimensional aspects of motor structure. Advantages are recognized from the use of materials such as a soft magnetically permeable medium that is amenable to formation of a variety of particular shapes. For example, core material may be manufactured from soft magnet grades of Fe, SiFe, SiFeCo, SiFeP powder material, each of which has a unique power loss, permeability and saturation level. Core geometries and core dimensions of stator elements, with relevant tolerances, can be formed without the need to form laminations and thus optimize the magnetic potential gradient developed between coupled poles of rotor permanent magnets and stator electromagnets. A structural configuration is disclosed wherein axially aligned stator poles and axially aligned rotor magnets provide highly concentrated flux distribution. Such configuration provides a greater number of poles with the same individual active air gap surface areas and/or greater total active air gap surface area than conventional motors having the same air gap diameter.
In summary, concentration of flux, maximization of flux, minimization of flux loss and transformer interference effects, are all contributing factors in the attainment of efficient motor operation with high torque capability. Motor structural configurations in which multiple poles are in axial alignment to provide efficient operation at high torque output have been described in the above-identified copending applications. Such arrangements, due to the relatively great volume occupied by the large number of stator core elements and rotor poles, are advantageous for use in environments in which space and weight considerations are not at a premium. There is a continuing need for motor structural configurations that provide these improved attributes as well as economy of size and geometry.
The present invention furthers the above-described needs of the prior art and provides additional advantages for configurations such as the isolated individual pole pair arrangements disclosed in the above identified Maslov et al. applications. Advantages of the present invention are achieved, at least in part, by further development of motor structural configurations to increase the surface areas of opposing stator poles and rotor poles across a plurality of air gaps. The relatively larger surfaces in which flux can be concentrated promote high torque capacity.
The structural features of one such configuration of the invention are embodied in a motor that comprises a rotor and a stator comprising a plurality of separate electromagnet core segments disposed coaxially about an axis of rotation. The stator core segments form an annular stator ring bounded by an inner and outer diameter. The core segments are affixed, without ferromagnetic contact with each other, to a non-ferromagnetic support structure. The rotor is configured in an annular ring that at least partially surrounds the annular stator to define two parallel axial air gaps between the rotor and stator respectively on opposite axial sides of the stator. Permanent magnets are distributed on each side of the rotor annular ring that faces an air gap. Each stator electromagnet core segment comprises a pair of poles aligned in a direction generally parallel to the axis of rotation with pole faces generally perpendicular to the axis of rotation. A winding is formed on a core portion that links the poles to produce, when energized, magnetic poles of opposite polarity at the pole faces. A change in the direction of current effects a reversal of the magnetic polarities.
In a preferred embodiment, the rotor annular ring has a U-shaped cross section with two side portions formed of ferromagnetic material connected by a cross portion. The cross portion may also be formed of ferromagnetic material. The permanent magnets are spaced from each other and distributed along the inner surfaces of the side portions. The cross portion of the rotor is radially separated from the outer diameter of the stator. Each relatively flat permanent magnet is a magnetic dipole having one magnetic polarity at its surface facing the air gap and the opposite magnetic polarity at its surface mounted to the side portion. The permanent magnets thus have magnetic polar orientation in the axial direction. The permanent magnets on each side portion successively alternate in polarity along the rotor ring and are separated from direct contact with each other. The number of permanent magnets on the two side portions are equal to each other and are respectively in axial alignment with each other with opposite magnetic polarities to permit appropriate motor drive interaction with oppositely magnetized stator pole pair faces.
Flux distribution can be improved even further by providing additional permanent magnets mounted on the side portions in the spaces between the adjacent axially oriented permanent magnets. The thickness dimension of the additional magnets in the radial direction may be of the same order as the motor drive permanent magnets. The magnetic polar orientations of the additional permanent magnets are in directions perpendicular to the axial direction. The effect of the additional magnets is to concentrate flux through the paths of the drive magnets, thus minimizing stray flux while decreasing the size of the back iron.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.